Fiber optic modules interface optical fibers to electronic circuitry transducing communication by light or photons with communication by electrical signals. A fiber optic module may be a fiber optic receiver, transmitter or transceiver including both receive and transmit functions. The fiber optic receiver, transmitter and transceiver each have optical elements (OE) and electrical elements (EE). The fiber optic transmitter OE includes an emitter (such as a semiconductor LED or Laser) mounted in a package and an optical coupling element for coupling light or photons from the OE into the optical fiber. The type of semiconductor laser (light amplification by stimulated emission of radiation) may be a vertical cavity surface emitting laser (VCSEL). The fiber optic receiver OE includes a photodetector (such as a photodiode) mounted in a package and an optical coupling element for coupling light or photons from the optical fiber into the photodetector. The EE for each includes integrated circuits and passive elements mounted on a substrate such as a printed circuit board (PCB) or ceramic. The OE and EE are connected electrically at the emitter and photodetector.
Because of the high transmission frequencies utilized in fiber optic communication, crosstalk between receive and transmit signals is of concern. Additionally, electromagnetic interference (EMI) is of concern due to the high frequency of operation of the fiber optic modules. In order to reduce EMI, shielding of the electrical components is required which is usually accomplished by attaching a metal shield to the substrate of the fiber optic module and connecting it to ground. In order to avoid electronic crosstalk and EMI, the fiber optic transceiver usually employs separate components and separate shielding of fiber optic receiver and fiber optic transmitter components. In order to avoid optical crosstalk where light or photons can interfere between communication channels, the fiber optic transceiver usually employs separate optical elements for coupling light or photons into and out of the optical fiber for fiber optic receiver and fiber optic transmitter. Using separate optical elements requires additional components and increases the costs of fiber optic transceivers. It is desirable to reduce the component count of fiber optic transceivers such that they are less expensive to manufacture.
The form factor or size of the fiber optic module is of concern. Previously, the fiber optic transceiver, receiver, and transmitter utilized horizontal boards or substrates which mounted parallel with a system printed circuit board utilized significant footprint or board space. The horizontal boards provided nearly zero optical crosstalk and minimal electronic crosstalk when properly shielded. However, the horizontal boards, parallel to the system printed circuit board, required large spacing between optical fiber connectors to make the connection to the optical fibers. While this may have been satisfactory for early systems using minimal fiber optic communication, the trend is towards greater usage of fiber optic communication requiring improved connectivity and smaller optical fiber connectors to more densely pack them on a system printed circuit board. Thus, it is desirable to minimize the size of system printed circuit boards (PCBs) and accordingly it is desirable to reduce the footprint of the fiber optic module which will attach to such system PCBs. Additionally, the desire for tighter interconnect leads of fiber optic cables, restricts the size of the OE's. For example, in the common implementation using TO header and can, the header dimension of the interconnect lead is normally 5.6 mm. In small form factor optical modules, such as the MT family, the two optical fibers are separated by a distance of only 0.75 mm. This severely restricts the method of coupling light or photons from the OE into and out of fiber optic cables.
There are a number of types of fiber optic cables available. The types of fiber optic cables can vary by the mode or the frequencies supported (single or multimode), the diameter of the fiber, the type of index of refraction (graded, stepped, uniform, etc.), and other factors. Often times the received light from an optical fiber is nonuniform making the alignment between optical fiber and an optical element more critical. Additionally, the light output from a light transmitter is often a single mode or only having a couple modes and it is desirable to excite multiple modes in a multimode optical fiber. When multiple modes are coupled into a multimode fiber, photons propagate at different speeds in the fiber. Such a difference in speed causes an effect known as the differential mode delay (DMD) phenomenon in multimode optical fibers which can reduce the optical transmission distance within an optical fiber. To overcome this phenomenon, the beam is ordinarily off-center launched into the fiber to excite only higher order modes of the fiber. Fewer modes in the fiber will reduce the DMD effect. However, such a launch technique increases alignment difficulty in the assembly, and thus the cost of the module. It is therefore desirable to have a single design that can couple light into or receive light from various diameters of optical fibers with better alignment tolerance. Thus, it is desirable to improve the OEs of fiber optic modules for coupling or launching light into various fiber optic cables and for receiving light from various fiber optic cables.